2.2 Measuring earthquakes

---

Vocabulary

---

focus-the point below the earths surface where rocks under stress break causing an earthquake
epicenter-the point directly above the epicenter
seismic waves-vibrations in the earth carrying energy during an earthquake
P waves-waves that compress and expand the ground
S waves-waves that vibrate side to side and up and down
surface waves-waves that reach the surface
seismograph-devices that records the vibrations of seismic waves
magnitude-a measurement of earthquake strength based on seismic waves and movement along faults
Mercalli scale-a measurement that was developed to rate the intensity of earthquakes
Richter scale-a rating o the size of seismic waves
moment magnitude scale- a rating system that estimates the total energy released by an earthquake

Outline

---

Seismic waves

Primary waves

• compress and expand the ground
  • can travel trough water
• Secondary waves
  • moves the ground back and forth so as up and down
    • shake structures violently when they reached the surface
• surface waves
  • happens when P and S waves reach the surface
    • shakes the gound most violently

Locating the epicenter

• The farther from the epicenter, the greater time of the arival of P waves and S waves